I recently started having hot water problems. All of a sudden the water went from hot to warm. The hot water heater was 13 years old so I replaced it. Now the new heater still has no hot water. The pilot is lit, the gas is on, the water supply valve is open the tank is filling but there is still no hot water... Please help. I am about to lose my mind.

You turned the gas valve from pilot to on and turned the control setting up so the burner fired and can see/hear the burner?

If the burner is on but the tank is still filling you will need to wait. Example if the tank is a 40 gal. with a 37,000 btu/hr burner. With say 50-55 degree incoming ground/municipal water. You need to raise the temp. 70 degrees to get a 120-125 degree shower. That means 40 gals. X 8.34lbs per gal. = 333.6lbs. X 70 degree rise = 23,352 btus needed / 37,000 = .63hrs or 37.8 minutes to heat the tank.